It is commonplace in the well completion art to provide "centralizers" at spaced intervals along the length of a section of well casing to prevent the casing from directly contacting the wellbore. More particularly, in the completion of oil and gas wells, and particularly in formations which are not well consolidated, it is commonplace to "cement" the well by pumping concrete around the casing of the well so as to fill the space between the casing and the formation. This has the effect of sealing the wellbore against fluid or gas migration, and consolidates the formation in the vicinity of the well, rendering the well much more stable. The casing and the cement are then "perforated" where it is desired to produce the well, by detonation of explosive charges within the casing at corresponding points.
It is somewhat unusual that the casing of a well is disposed in the precise center of the wellbore. Commonly, the wellbore is not truly vertical, either due to unavoidable influences in the drilling operation or by design. Therefore, when the casing is lowered into the well, one side of the casing normally rests against the lower side of the wellbore. When the well is subsequently cemented, an insufficient amount of cement is pumped between this part of the casing and the wellbore, and the annular space therebetween is not suitably sealed at this point.
To avoid this problem, so-called "centralizers" are commonly employed. Centralizers as conventionally employed usually consist of two steel collars spaced from one another along the casing by on the order of two feet. Six or eight straps of relatively heavy gauge spring steel which are bent outwardly in a gentle bow shape are welded between the two collars. The centralizers may slide along the length of the casing between the couplings which join sections of the casing, or may be fixed thereto by means of "stop collars" setscrewed to the casing. Such centralizers provide "standoff", that is, a locally increased diameter for the casing, spacing the remainder of the casing from the wellbore. S centralizers are very commonly used in well completion in weakly consolidated or unconsolidated formations, to ensure that cement forms a complete annulus between the casing and the wellbore.
While the use of centralizers as thus described is entirely conventional, there are some inherent problems with use of these devices. Such centralizers are of larger diameter than the remainder of the casing, which can lead to difficulty in passing the casing down the wellbore with the centralizers attached. More specifically, the spring steel bows of centralizers intended to fit on "downhole" sections of casing are normally formed to provide a local outside diameter greater than the inside diameter of a section of "uphole" casing, and greater than the diameter of sealing surfaces provided by conventional casing "hangers". The centralizer bows can damage these sealing surfaces while the casing is lowered into the wellbore; in some cases the centralizers must be eliminated. It would be preferable if effective centralizers could be provided which when lowered are of no greater diameter than the couplings which join the sections of pipe making up the casing, but which can be caused to assume a larger diameter when in the well, forcing the casing toward the center of the wellbore.